This invention relates to the hot rolling of metals, especially steel. It relates particularly to the hot rolling of rail and other elongated steel sections and shapes.
In the hot rolling of steel rails, a red-hot bloom is passed through a sequence of rolls which gradually reshape the hot steel into a standard railroad rail section having a head, web and base. The rails which are rolled at temperatures well over 1800.degree. F. are then cooled under controlled conditions to prevent distortion of the rail and the formation of internal defects. Finished steel rail is subjected to very stringent specifications and inspections. The finished rail must conform to dimensional specifications that permit variations of only 1/32 of an inch or less.
During the hot rolling of the rail, the operator regularly checks samples of the finished rail to determine whether it meets the required dimensional specifications since the shaping rolls are constantly being worn or can get out of adjustment during the rolling. For the past 120 years, the roller for sampling, has cut a short section from the end of a hot finished rail and rapidly cooled the sample by plunging it into a water tank thereby cooling it down to ambient temperature within a few minutes. The roller then checked the dimensions of the cooled sample with a gage to determine whether it met specifications. If not, the roller made adjustments in the settings of the shaping rolls to correct for the dimensional variations based on the measurements of the cooled sample.
It has been discovered that many of the samples taken during the hot rolling of steel rail do not provide accurate information concerning the dimensions of the finished rail due to the fact that the austenite in the hot sample was rapidly changed to martensite during the rapid quench in water whereas the elongated finished rail was allowed to cool more slowly to form a desirable pearlitic structure. Steels that contain alloy additions to increase the hardness of the steel often change dimensions if not cooled properly. As a result, significant dimensional differences often existed between the rapidly cooled sample and the slowly cooled finished rail section.